While packet switched services are currently on the rise, transport networks today still rely mainly on circuit switched technology such as SDH (Synchronous Digital Hierarchy), where constant rate transport signals transport time-division multiplexed subrate signals. Payload signals are mapped into multiplex units, which in SDH are termed virtual containers. A multiplex unit represents an end-to-end path through the network and connections in the network are established by configuring the network elements to semi-permanently switch the corresponding multiplex unit, which is present at the same relative position in all consecutive transport frames, along that path.
Network elements are for example add/drop multiplexers and digital crossconnects. Such network elements contain I/O (input/output) ports and a switch matrix interconnecting the I/O ports. The switch function which needs to be performed on the level of multiplex units to establish paths in the network encompasses switching in both, space and time domain, i.e. between different I/O ports and between different timeslot positions. Typically, the transport signals are retimed and aligned at the input port and the switch function is performed on time slots of the aligned signal in synchronism with a common system clock in accordance with a pre-configured interconnection map.